1. Field of the Invention
The present invention relates to polymeric compositions that can be used to stabilize reactive clays in subterranean formations and to a method of stabilizing such reactive clays.
2. Description of the Prior Art
In the drilling of oil and gas wells and other such boreholes, it is generally necessary to use a drilling fluid that performs a variety of functions but is primarily used to lubricate the drill bit and carry the cuttings to the surface. It is not uncommon to encounter formations during the drilling process that are argillaceous. These argillaceous formations contain clay (shale) particles, the proportion of which can vary over a wide range. When these argillaceous formations are encountered, and if a water based drilling fluid is employed, complex chemical reactions take place within the argillaceous formation, including ion exchange, hydration, etc. These reactions result in swelling, crumbling or dispersion of the clay or shale particles in the formation through which the drill bit passes. Indeed, the problems can become so severe that washout and even complete collapse of the borehole can occur.
In an attempt to solve these problems, additives are included in the aqueous drilling fluid that are designed to arrest or slow down fluid invasion into the formation by forming an impermeable or semi-impermeable membrane on the surface of the borehole. For example, sodium silicate-based drilling fluids are believed to form such semi-impermeable membranes and are commonly used for maintaining shale stability. Other drilling fluid systems have also been used to deal with the problem of shale stabilization. For example, U.S. Pat. No. 4,299,710 discloses a drilling fluid comprising an aqueous solution of a combination of thickeners such as a copolymer and a polysaccharide. Additionally, U.S. Pat. No. 5,972,848 discloses a method for stabilizing shale formations by employing an aqueous solution containing a polymer with a hydrophilic group and a hydrophobic group.